The invention relates to materials for implantation which are based on polyacrylates and which can be used, in particular, as materials for replacement of bone, for uniting bone and for anchoring prostheses.
The admixture of absorbable tricalcium phosphate to these materials for implantation, which are also known as bone cements, is disclosed, in European Pat. No. A1 16906, in order to provide in the implant, by the onset of absorption at the margin of the implant, channels into which the bone tissue can grow. This leads to the implant meshing together with the surrounding bone and thus to improved long-term stability. By use of relatively small amounts of tricalcium phosphate, of from 5 to 35 percent by weight, in particular about 20 to 25 percent by weight, there is essentially the development of only marginal porosity, since the absorbable particles of filler in the implant are not so closely packed that they come into contact with one another, and thus complete absorption starting from the margin would be possible.
It has now emerged that the particles of filler located inside the implant can be the starting point for disintegration of the implant. Apparently, the particles of tricalcium phosphate do not participate in elastic movements of the acrylate polymer, which take place when the prosthesis which has been cemented in is subjected to loading and unloading, so that stress situations arise at the incorporated particles and, in the long term, these destroy the structure of the polymer.
A solution to this problem taking the form of increasing the proportion of filler to such an extent that the particles come into contact with one another and thus can be completely absorbed starting from the margin is inadvisable since this would lead to a great reduction in the short-term stability of the implant itself. On the other hand, with a marginal porosity the anchoring of the implant which is achieved is sufficiently good for it to be possible to dispense with bone tissue growing completely through the implant with the aim of good longterm stability.
Thus, the object was to find a filling material which is based on tricalcium phosphate and which can be admixed to the cement in the amounts and particle sizes disclosed in European Patent A1 16906 but which essentially avoids stress situations between particles of filler and the polymer composition.